System for mooring a production vessel

ABSTRACT

A system for mooring a floating production vessel comprising: two or more buoys anchored to the seafloor that are also connected directly to the floating production vessel; and one or more risers connected to at least one of the buoys that is also connected to one or more subsea wellheads or manifolds wherein the buoys are not connected to the floating vessel by a swivel and turret system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/610,680, filed Mar. 14, 2012, which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a system and method for mooring a production vessel comprising two or more buoys.

BACKGROUND

Floating production vessels are often used to capture oil and gas produced from deepwater production wells. Risers are used to carry oil and gas from the subsea wellhead to the floating production vessel. The vessels are typically moored in place to prevent excessive movement while the floating production vessel is connected to the risers.

Floating production vessels are typically connected to the risers using a swivel and turret system to allow the vessel to weathervane around the risers to stay aligned with the prevailing weather. These swivel and turret systems are complex and expensive, especially when used in high pressure and/or high temperature conditions. They also require intensive maintenance programs. In cyclonic or iceberg-prone environments, these turrets are typically disconnectable, adding to their complexity.

It would be advantageous to provide a disconnectable system for mooring a floating production vessel at a well-site in a less complicated way while still allowing for connection with the risers and passing of the oil and gas to the floating production vessel.

SUMMARY OF THE INVENTION

The invention provides a system for mooring a floating production vessel comprising: a) two or more buoys anchored to the seafloor that are also connected directly to the floating production vessel; and b) one or more risers connected to at least one of the buoys that is also connected to one or more subsea wellheads or manifolds wherein the buoys are not connected to the floating vessel by a swivel and turret system.

The invention also provides a method of mooring a floating production vessel comprising: providing two or more buoys that are anchored to the seafloor; and connecting the two or more buoys to the floating vessel wherein the buoys are not connected to the floating vessel by a swivel and turret system.

The invention further provides a method of protecting the floating vessel and the production risers from the effects of a storm comprising: a) providing two or more buoys that are anchored to the seafloor and connected to a floating production vessel; b) providing one or more production risers that are connected to a source of oil and gas production on the seafloor and to at least one of the buoys; c) disconnecting all buoys that are connected to the one or more production risers from the floating production vessel, protecting both the former and the latter from the effects of a storm; and d) submerging all buoys to a depth of at least 10 meters below sea level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of a floating production vessel moored to two buoys.

FIG. 2 depicts an embodiment of a floating production vessel moored to two buoys connected together.

FIG. 3 depicts an embodiment of a floating production vessel disconnected from two buoys.

FIG. 4 depicts an embodiment of a floating production vessel disconnected from two buoys connected together.

DETAILED DESCRIPTION

A floating production vessel preferred for use with the mooring system is a ship-shaped vessel or another elongated vessel. For example, the vessel can be an FPSO (floating production storage and offloading) vessel, an FSO (floating storage and offloading) vessel, an FLNG (floating liquefied natural gas) vessel or any other similar vessel that is used for capturing oil and gas from a subsea oil and gas well.

This vessel can be used in a number of ways, all of which are suitable for use with the mooring system claimed herein. The vessel can be used to process the oil and gas, store it for some period of time and then offload the oil and gas to another vessel or to a pipeline. Alternatively the vessel does not process the oil and gas, but it merely stores it and then offloads it to another vessel. In the example of an FLNG vessel, the natural gas from the well would be processed and liquefied onboard the vessel.

The floating production vessel is moored in place by connecting the vessel to two or more buoys. The buoys are anchored to the seafloor by mooring lines. The buoys are typically spread-moored, meaning that mooring lines extend out in an at least somewhat horizontal direction to provide horizontal resistance to movement of the buoys. The buoys are moored with at least two mooring lines. In one embodiment at least one of the buoys is moored with at least three mooring lines and in another embodiment at least one of the buoys is moored with at least four mooring lines. The mooring lines of the different buoys can be placed in such a manner as to provide an optimal resistance to horizontal movement of the floating production vessel when connected to the buoys.

The buoys are directly connected to the floating production vessel without the use of a swivel and turret system. Embodiments of methods of connecting and disconnecting the buoys from the floating production vessel will be described herein.

In conventional mooring systems, a swivel and turret system is used to provide weathervaning capability to the floating production vessel. The swivel and turret allows the vessel to rotate to be pointed in the direction of the oncoming environmental loads to reduce total load on the moorings. The swivel and turret systems can be external to the floating production vessel or internal to the floating production vessel. These systems are complex and expensive, as they must be designed to allow for the flow of oil and gas from the risers to the ship regardless of the direction or degree of rotation around the swivel and turret.

The ability to rotate completely around is not always a requirement for the floating production vessels, and the present invention provides a mooring system that is much simpler and less expensive than the conventional swivel and turret systems. In addition, the swivel and turret systems become harder to design and more complex when dealing with oil and gas at high pressure and/or high temperature.

The risers are used to transport the oil and gas from the subsea wells, manifolds or other subsea apparatus to the floating production vessel. Since fields usually have a plurality of wells and/or other subsea apparatus, a plurality of risers are required to transport the oil and gas.

The risers are connected in some fashion to the buoys so that the buoy can provide sufficient buoyancy to hold the weight of the riser. Risers may be connected to only one buoy. In another embodiment, risers are connected to two or more buoys. By connecting risers to more than one buoy, the capacity for risers can be increased. This is especially helpful when producing oil and gas at high temperature and/or high pressure because the risers are thicker and heavier as a result of the more difficult conditions. If a swivel and turret system were used, the number of risers would be limited to the capacity of the one swivel and turret system.

The risers may be connected to the buoys when they are installed at the well-site even if the floating production vessel is not at the well-site. The buoys provide buoyancy for the risers, but it is preferred for the buoys once connected to the risers to be at least partially submerged. It is more preferred for the buoys to be below the depth where wave action, shipping traffic, or iceberg passage will have a significant impact on the risers.

To connect the buoys with the floating production vessel, the buoys are preferably lifted into contact with the floating production vessel and connected in any suitable manner to maintain the connection between the buoy and the floating production vessel. In one embodiment, messenger lines may be used to connect to the buoys and then pull in the buoys into position under the vessel. The buoys are then pulled in further and secured onto the vessel.

Once the buoys are connected to the floating production vessel, the risers can be connected to the production and/or storage system on the floating production vessel and then the flow of oil and gas can be initiated.

In one embodiment, the buoys are connected to each other. This is helpful in maintaining the position of the buoys with respect to each other when they are not connected to the floating production vessel. The connecting line may be tensioned when the buoys are disconnected to help maintain the buoy positions.

Due to the nature of the spread-mooring system, the connected floating production vessel has a limited ability to move and rotate with the weather. In the event of any significant predictable threats such as hurricanes, cyclones, icebergs, etc. the floating production vessel can be disconnected from the buoys so that it can escape the threat. Once disconnected, the buoys are submerged to a depth below the zone of significant wave action. Later, the floating production vessel can be reconnected with the buoys and production can be reinitiated.

Alternatively, the buoys can be disconnected and submerged if the floating production vessel will be leaving the well-site. The buoys may be submerged to a depth of at least 10 m below the sea level, preferably at least 25 m below sea level and more preferably at least 50 m below sea level. The preferred depth for the buoys may be dependent on the conditions present at the well-site.

This system provides a disconnectable, low-cost, mechanically simple system for mooring floating production vessels involved in deepwater developments in hurricane/cyclone and iceberg-prone zones that can accommodate a larger number of risers than is possible with conventional systems. This additional riser capacity can also translate to the ability to use heavier and larger risers that are able to handle demanding high pressure and/or high temperature reservoir conditions.

An embodiment of the invention is depicted in FIG. 1. Floating production vessel 10 floats on the surface 50 of the ocean. The floating production vessel is connected to two buoys 20. The buoys are moored by mooring lines 30 that are anchored to the seafloor 60. In addition, risers 40 are connected to the buoys. Since the buoys 20 are connected to the floating production vessel 10, the risers can be connected to production and/or storage equipment on the vessel.

Another embodiment of the invention is depicted in FIG. 2. Everything is similar to that depicted in FIG. 1, except that in this embodiment, the two buoys are connected by connecting line 70. This can be any type of line connecting the two buoys. This line is especially useful when the floating production vessel disconnects from the buoys because it helps keep the buoys moving in unison.

FIG. 3 depicts the mooring system when the floating production vessel has disconnected from the buoys. This can happen, for example, when a hurricane, cyclone or iceberg is headed towards the well-site. The buoys are disconnected and submerged below the level of significant wave action and iceberg impact. The floating production vessel 10 can then move out of the path of the storm. The buoys and risers are protected from the storm and the resulting waves.

FIG. 4 depicts another embodiment of the mooring system when the floating production vessel has disconnected from the buoys. In this embodiment, the buoys are connected together by connecting line 70. The connecting line is preferably tensioned to help keep the buoys in place. 

1. A system for mooring a floating production vessel comprising: a. two or more buoys anchored to the seafloor that are also connected directly to the floating production vessel; and b. one or more risers connected to at least one of the buoys that is also connected to one or more subsea wellheads or manifolds wherein the buoys are not connected to the floating vessel by a swivel and turret system.
 2. A system as claimed in claim 1 wherein risers are connected to each of the two or more buoys.
 3. A system as claimed in claim 1 wherein the buoys can be disconnected from the floating vessel by releasing the buoys to be submerged at a deeper depth than that when connected.
 4. A system as claimed in claim 1 wherein each of the buoys is anchored to the seafloor by more than two mooring lines.
 5. A system as claimed in claim 4 wherein the two or more buoys are anchored to the seafloor by a plurality of mooring lines positioned at an angle to the vertical.
 6. A system as claimed in claim 1 wherein two or more buoys are connected to each other with one or more connection lines.
 7. A system as claimed in claim 1 wherein the floating production vessel is a ship.
 8. A system as claimed in claim 1 wherein the two or more buoys are not connected directly to each other.
 9. A method of mooring a floating production vessel comprising: a. providing two or more buoys that are anchored to the seafloor; and b. connecting the two or more buoys to the floating vessel wherein the buoys are not connected to the floating vessel by a swivel and turret system.
 10. A method as claimed in claim 9 wherein the buoys are connected to each other with one or more connection lines.
 11. A method as claimed in claim 9 wherein the two or more buoys are not connected directly to each other.
 12. A method of protecting production risers and the floating production vessel from the effects of a storm comprising: a. providing two or more buoys that are anchored to the seafloor and connected to a floating production vessel; b. providing one or more production risers that are connected to a source of oil and gas production on the seafloor and to at least one of the buoys; c. disconnecting all buoys that are connected to the one or more production risers from the floating production vessel; and d. submerging all buoys to a depth of at least 10 meters below sea level.
 13. A method as claim in claim 12 further comprising moving the floating production vessel away from the storm. 